Electronic properties of doped fullerenes

Forró, Lászlø; Mihály, L.

doi:10.1088/0034-4885/64/5/202

Cited by 228 publications

(164 citation statements)

References 252 publications

(363 reference statements)

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“…Analysis of a simple model (1) revealed a stunning variety of phases in the immediate neighborhood of the Mott metal-insulator transition near half filling. The variety is greater than either that predicted for the three band case fullerides [14], or that experimentally known in fullerides [2] as well as in doped organics (with lower symmetry than MPcs) near half filling [1]. Many of the phases described above will individually merit an in-depth study.…”

Section: Pacs Numbersmentioning

confidence: 85%

Strong Correlations in Electron Doped Phthalocyanine Conductors Near Half Filling

et al. 2004

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We propose that electron doped nontransition metal-phthalocyanines (MPc) like ZnPc and MgPc, similar to those very recently reported, should constitute novel strongly correlated metals. Due to orbital degeneracy, Jahn-Teller coupling and Hund's rule exchange, and with a large on-site Coulomb repulsion, these molecular conductors should display, particularly near half filling at two electrons/molecule, very unconventional properties, including Mott insulators, strongly correlated superconductivity, and other intriguing phases. PACS numbers:Building novel metals by doping molecular crystals such as polyacetylene, fullerenes, TTF-TCNQ, (TMTSF) 2 X, (TMTTF) 2 X and (BEDT-TTF) 2 X salts, etc., is a well trodden route [1,2], but remains an exciting and ever moving front. Very recently the Delft group showed that thin films of transition metal phthalocyanines (MPcs) FePc, CoPc, NiPc, CuPc, initially insulating, can be turned genuinely metallic through potassium doping [3]. Electron doping appears to take place largely in the twofold degenerate lowest unoccupied e g molecular orbital (LUMO) [4,5] of the molecules. The simplest rigid band model naturally explains, according to [3], why the MPcs, initially insulating when pristine (n = 0) [6], become metallic upon increasing doping (0 < n < 4), ending up again as insulators at full doping (n = 4). It is not yet clear whether stoichiometric compound phases may exist here as they do in alkali doped fullerides, but there is an otherwise striking overall analogy, to the point that even the conductance values reported at optimal metallic doping are close in magnitude and temperature (in)dependence to those of K x C 60 films.While the pursuit of metallicity-cum-magnetism in the transition metal doped MPcs [5] will in the future constitute an interesting goal in itself, the scope of this note is to point out newer directions and possibilities that can make slightly different metallic doped MPcs, yet to be realized, potentially even more exciting. Doped MPcs are not, we claim, regular metals, but constitute strongly correlated electron systems, akin to doped two-band Mott insulators. Characterized by a strong on-site electron repulsion, by the e g orbital degeneracy, and by intra-site Jahn-Teller and electron-electron multiplet couplings, the doped MPc molecular system should approach near n = 2, as foreshadowed by recent studies [7,8,9] a novel unstable fixed point heralding a wealth of possible low-temperature phenomena and phases. We conduct in this letter a preliminary exploration of this scenario by addressing theoretically -and thus proposing the experimental realization of -new metals obtained through electron doping of ZnPc, MgPc and other such non-transition metal MPcs. The extra electrons should flow into the MPc 2e g lowest unoccupied molecular orbital (LUMO) [4], to form a two band metal. The LUMO states are ligand shell orbitals loosely surrounding the central metal ion, with a large intramolecular Coulomb repulsion U in comparison with the narrow electron bandwidth...

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Section: Pacs Numbersmentioning

confidence: 85%

Strong Correlations in Electron Doped Phthalocyanine Conductors Near Half Filling

et al. 2004

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“…5 Thus the electronic structure of the outer shell can be discussed in complete analogy with p electrons on an atom. 2 In any uniaxial crystal field, one expects a twofold orbital splitting to e 2u and a u , whereas in a biaxial system the degeneracy is completely lifted, resulting in three distinct electronic levels. ͓It follows from the SO͑3͒ symmetry that the principal axes of the distortions can be taken as the axes of the crystallographic unit cell, regardless of the position of the molecule within the crystal.͔…”

mentioning

confidence: 99%

Ordered low-temperature structure inK4C60detected by infrared spectroscopy

et al. 2002

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Infrared spectra of a K 4 C 60 single-phase thin film have been measured between room temperature and 20 K. At low temperatures, the two high-frequency T 1u modes appear as triplets, indicating a static D 2h crystal-field stabilized Jahn-Teller distortion of the C 60 4Ϫ anions. The T 1u (4) mode changes into a known doublet above 250 K, a pattern which could have three origins: a dynamic Jahn-Teller effect, static disorder between ''staggered'' anions, or a phase transition from an orientationally ordered phase to one where molecular motion is significant. The electronic structure of fulleride salts has been a topic of intense interest since their discovery. Three recent reviews summarized the situation: Reed and Bolskar 1 focused on isolated ions; Forró and Mihály 2 treated fulleride solids; and Gunnarson 3 concentrated on the theory of superconductivity and on band-structure calculations, including the important concepts of the Jahn-Teller ͑JT͒ effect. 4 Fulleride anions with charges other than 0 or 6 are susceptible to Jahn-Teller coupling between electrons occupying partially filled t 1u orbitals and H g phonons. Because of the high degeneracy of the phonons involved, the coupled electronic Hamiltonian acquires an SO͑3͒, higher than the original icosahedral configuration. 5 Thus the electronic structure of the outer shell can be discussed in complete analogy with p electrons on an atom. 2 In any uniaxial crystal field, one expects a twofold orbital splitting to e 2u and a u , whereas in a biaxial system the degeneracy is completely lifted, resulting in three distinct electronic levels. ͓It follows from the SO͑3͒ symmetry that the principal axes of the distortions can be taken as the axes of the crystallographic unit cell, regardless of the position of the molecule within the crystal.͔ In solids the orbital splitting is small compared to the bandwidth; thus electron correlation has to be taken into account. The most comprehensive theoretical treatment of this interplay was given by Fabrizio and co-workers, who introduced the concept of the ''Mott-Jahn-Teller insulator'' ͑M-JT͒. 6,7 The model of p-like electrons on a spherical or ellipsoidal surface was indeed succesful in the interpretation of NMR, 8,9 photoelectron spectroscopy, 10,11 resonance Raman, 12 and magnetic 13 data. The situation becomes different, however, once we try to measure properties that reflect the internal degrees of freedom within the C 60 ball itself. Typical examples are magnetic resonance measurements and vibrational spectroscopy. When atomic motion has to be taken into account, the symmetry can no longer be regarded as spherical. Instead, the actual shape of the molecule is determined by the combined internal ͑JT͒ and external ͑crystal field͒ distorting forces. If these result in a potential surface with several shallow wells, then the balls will assume different shapes and positions with respect to the lattice. The result will be either static disorder or random fluctuations between the minima, called the dynamic Jahn-Teller effect....

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“…Prominent analytic deviation from respective non-dipole parameters for inelastic scattering, given by (20,21,22,24) is clearly seen. Contrary to the dipole parameters, simple frequency independent relations that connect respective non-dipole parameters for photoionization and fast electron inelastic scattering do not exist.…”

Section: Angular Anisotropy Parameters For P and D-subshellsmentioning

confidence: 94%

“…Of additional interest are the recently discovered [20] onion-type endohedrals. In these objects inside a very big fullerene R is valid.…”

Section: Onion-type Fullerenesmentioning

confidence: 99%

Generalized oscillator strength of endohedral molecules

Amusia

Чернышева

Liverts

2012

Int J of Quantum Chemistry

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We investigate here the fast electron scattering upon endohedral atoms that present a fullerene C N staffed by an atom A, A@C N . We calculate the inelastic scattering cross-section expressing it via generalized oscillator strengths (GOS) density. We take into account two major effects of C N upon ionization of the atom A. Namely, the scattering of the electron by the static potential of the fullerenes shell and modification of the interaction between the fast incoming and atomic electrons due to polarization of the fullerenes shell by the incoming electron beam.To obtain the main features of the effect, we substitute the complex fullerenes shell C N by a static zero thickness potential, express its deformation under incoming electron action via C N polarizabilities. We perform all consideration in the frame of the so-called random phase approximation with exchange (RPAE) that gave reliable results for GOSes of isolated atoms, and expressions for absolute and differential in angle cross-sections. We limit ourselves with dipole and biggest non-dipole contributions to the differential in angle cross-section.We compare fast electron scattering with photoionization as a source of information on the target electronic structure and emphasize some advantages of fast electron scattering. As concrete objects of calculations, we choose noble gas endohedrals Ar@C 60 and Xe@C 60 . The results are presented for two transferred momentum q values: q=0.1 and 1.0. Even for small q=0.1, in the so-called optical limit, the deviation from photoionization case is prominent and instructive.As an interesting and very specific object, we study onion-type endohedrals A@C N1 @C N2 , in which the all construction A@C N1 is stuffed inside C N2 with N 2 >> N 1 .

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Electronic properties of doped fullerenes

Cited by 228 publications

References 252 publications

Strong Correlations in Electron Doped Phthalocyanine Conductors Near Half Filling

Strong Correlations in Electron Doped Phthalocyanine Conductors Near Half Filling

Ordered low-temperature structure inK4C60detected by infrared spectroscopy

Generalized oscillator strength of endohedral molecules

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